diff --git a/src/Registry/AppRegistry.php b/src/Registry/AppRegistry.php
index a0db806..f88d0ae 100644
--- a/src/Registry/AppRegistry.php
+++ b/src/Registry/AppRegistry.php
@@ -47,6 +47,11 @@ public function has(string $appName): bool
 
     public function get(string $appName): AppInterface
     {
+        if(!$this->has($appName)) {
+            throw new \InvalidArgumentException(
+                sprintf('App "%s" does not exist', $appName)
+            );
+        }
         return $this->apps[$appName];
     }
 }
diff --git a/src/Registry/ManifestRegistry.php b/src/Registry/ManifestRegistry.php
index ac3e938..36fd469 100644
--- a/src/Registry/ManifestRegistry.php
+++ b/src/Registry/ManifestRegistry.php
@@ -23,25 +23,27 @@ public function __construct(iterable $manifests)
     /**
      * @param AppInterface $app
      *
+     * @param string|null $className
      * @return iterable|ManifestInterface[]
      */
-    public function byApp(AppInterface $app): iterable
+    public function byApp(AppInterface $app, string $className = null): iterable
     {
         $reflection = new \ReflectionObject($app);
 
-        return $this->byNamespacePrefix($reflection->getNamespaceName());
+        return $this->byNamespacePrefix($reflection->getNamespaceName(), $className);
     }
 
     /**
      * @param string $namespacePrefix
      *
+     * @param string|null $className
      * @return iterable|ManifestInterface[]
      */
-    public function byNamespacePrefix(string $namespacePrefix): iterable
+    public function byNamespacePrefix(string $namespacePrefix, string $className = null): iterable
     {
         foreach ($this->manifests as $manifest) {
             $class = get_class($manifest);
-            if (str_starts_with($class, $namespacePrefix)) {
+            if (str_starts_with($class, $namespacePrefix) && (null === $className || $manifest instanceof $className)) {
                 yield $manifest;
             }
         }
diff --git a/src/Registry/ProjectRegistry.php b/src/Registry/ProjectRegistry.php
index 3e1b1fa..1b4e9a3 100644
--- a/src/Registry/ProjectRegistry.php
+++ b/src/Registry/ProjectRegistry.php
@@ -51,6 +51,11 @@ public function has(string $projectName): bool
 
     public function get(string $projectName): ProjectInterface
     {
+        if(!$this->has($projectName)) {
+            throw new \InvalidArgumentException(
+                sprintf('Project "%s" does not exist', $projectName)
+            );
+        }
         return $this->projects[$projectName];
     }
 }